1. Technical Field
The present invention relates generally to passive optical networks and more particularly to passive optical networks utilizing a sub-carrier multiplexing scheme.
2. Description of the Related Art
Passive Optical Network (PON) systems are commonly employed to provide high-bandwidth-guaranteed services and high-definition video services, among other services. Typically, passive optical networks incorporate a point-to-multipoint architecture in which a single optical fiber may be used to service multiple users. Examples of passive optical network architectures include time division multiplexing (TDM) networks, wavelength division multiplexing (WDM) networks, and sub-carrier multiplexing networks (SCM), each of which is discussed briefly below.
TDM Networks
With reference to FIG. 1, an exemplary TDM-PON system 100 is illustrated. The TDM-PON system 100 may include a plurality of optical network units (ONUS) 102-1, 102-2, 102-3, situated at client locations for the upstream transmission of data streams 104-1, 104-2, 104-3, respectively, and an optical line terminal (OLT) 106 for receiving the data that may include a receiver 108 and a time de-multiplexer 110. In conformance with current PON architectures, the common fiber feeder of the PON is shared by all the ONUs terminating the branching fibers. Transmission of upstream traffic from the ONUs to the OLT generally entails incorporating accurate multiple access techniques so that the traffic streams generated by the ONUs are multiplexed in a collision-free way onto the common feeder fiber.
As illustrated in the TDM-PON of FIG. 1, the upstream packets from the ONUs are time-interleaved at a power splitting point 105, which involves careful synchronization of the packet transmission instants at the ONUs. This synchronization is achieved by means of grants sent from the OLT, which instruct the ONUs when to send a packet. The correct timing of these submissions may be achieved by ranging protocols, which sense the distance from each optical network unit (ONU) to the OLT. In the OLT, a burst mode receiver 108 is utilized to timely synchronize packets originating from different ONUs. The burst mode receiver 108 also may manage amplitude levels of the packets, which may vary due to differences in path loss experienced by packets originating from different ONUs.
While a single receiver may be employed in a TDM-PON system, the TDM-PON system suffers from a disadvantage in that transmission capacity per ONU is limited. Because the ONUs jointly share the capacity of the OLT, the average data transmission capacity per ONU decreases as the number of ONUs increases.
WDM Networks
In contrast to TDM-PON systems, the transmission rate per ONU in WDM-PON systems is generally not dependent on the number of ONUs. A WDM-PON enables each ONU to occupy the bandwidth of a single optical wavelength, which may permit transmission up to 10 Gb/s or higher.
With Reference to FIG. 2, an exemplary WDM-PON system 200 is depicted. WDM-PON system 200 may include ONUs 202-1, 202-2 and 202-3, which transmit data streams 204-1, 204-2 and 204-3, respectively. In WDM-PON systems, m subscribers use m wavelengths to transmit upstream data simultaneously. In the example provided in FIG. 2, m is three, corresponding to three ONUs and three wavelengths, λ1, λ2 and λ3. Thus, ONUs 202-1, 202-2 and 202-3, transmit data along wavelengths λ1, λ2 and λ3, respectively, to a WDM multiplexer 206, which may comprise an Array Waveguide Grating (AWG).
The m upstream signals may be aggregated by the WDM multiplexer 206 and transmitted through a single optical fiber to an OLT 214 in a central office. The OLT 214 includes a WDM demultiplexer 208, which may comprise another AWG, configured to de-multiplex the upstream signals into multiple wavelength-dependent signals received by m optical receivers, 210-1, 210-2 and 210-3. The receivers 210-1, 210-2 and 210-3 extract data streams 212-1, 212-2 and 212-3 originating from ONUs 202-1, 202-2 and 202-3, respectively.
Thus, in a WDM-PON system, the wavelength channels may be routed between the OLT and ONUs in both directions by a wavelength demultiplexing/multiplexing device located at the PON splitting point, such as WDM multiplexer 206. Each ONU utilizes a different wavelength channel, for example, λ1, λ2 and λ3, to send its packets to the OLT and, as a result, the OLT is required to employ a WDM de-multiplexer 208 along with a receiver array, for example, 210-1, 210-2 and 210-3, to receive and process the upstream signals. The wavelength channels constitute independent communication channels on which different signal formats may be implemented. Furthermore, time synchronization between the channels is not needed to differentiate between data streams originating from different ONUs.
WDM-PON offers one solution for multiple accesses that creates a virtual point-to-point topology on a physical point-to-multipoint topology. Thus, in analogy with point-to-point system concepts, the WDM-PON concept facilitates scaling toward larger numbers of ONUs and enables simple service upgrading per individual customer.
However, current WDM-PONs are required to have one AWG and multiple receivers at the OLT to receive upstream data from multiple optical network units, which consequently increases the expense of the OLT. Therefore, due to the additionally required wavelength-selective functions (e.g., multiple wavelength dependent receivers and an AWG) at the OLT, WDM-PON is a costly solution. In addition, current WDM-PON systems lack flexibility in that they are incapable of dynamically allocating bandwidths to the ONUs. For example, when one ONU goes off-line, its bandwidth cannot automatically be assigned to another ONU.
Comparison of WDM-PONs and TDM-PONs illustrates that there is currently a trade-off between OLT cost and overall capacity. The OLT with a single receiver, as utilized in TDM-PON, is relatively low cost, but can only provide a single wavelength capacity without scalability. In contrast, in WDM-PON systems, the OLT can provide a larger capacity, but its cost and complexity are much higher due to its receiver array.
SCM Networks
With reference to FIG. 3, a subcarrier modulation (SCM) WDM-PON system 300 is illustrated. System 300 may include a plurality of ONUs 302-1, 302-2 and 302-3, which respectively transmit data streams 304-1, 304-2 and 304-3 along wavelengths λ1, λ2 and λ3, as described above with respect to WDM-PON system 200. However, unlike PON systems described above, the various ONUs modulate their packet streams on different electrical carrier frequencies and subsequently modulate the light intensity of their respective laser diodes. The packet streams are thus put into different electrical frequency hands. Each frequency band constitutes an independent communication channel transmitted from an ONU 302 to an OLT 316, and thus may carry a signal in a format different from that in another channel. Additionally, no time synchronization of the channels is needed.
The laser light modulated by the ONUs is multiplexed at WDM multiplexer 306 and transmitted to a photo detector 308 at the OLT 316, which converts the modulated light into modulated electrical signals within the different electrical frequency bands. The different electrical frequency bands are de-multiplexed at the OLT 316 by a frequency demultiplexer 310 and are transmitted to electrical receivers 312-1, 312-2 and 312-3. Data streams 314-1, 314-2 and 314-3, originating from ONUs 302-1, 302-2 and 302-3, respectively, are extracted by electrical receivers 312-1, 312-2 and 312-3, respectively.
While SCM-PON architectures can reduce the number of photo detectors employed to one, they require multiple electrical receivers. In addition, the electrical receivers need different band-pass filters to separate the different frequency bands and perform the frequency and time synchronization individually with each corresponding carrier frequency. To achieve the flexibility and scalability of dynamic capacity sharing among all ONU-s, tunable band-pass filters are required and the local oscillator is required to have the ability to synchronize with different carrier frequencies. Thus, although the SCM-PON may provide simultaneous wavelength-based transmission and dynamic bandwidth control flexibility within the same system, the analog components in the OLT increases the system complexity and cost significantly.
Although various existing PON systems have different advantages, as described above, existing PON systems do not meet current demands for providing cost efficient optical transmission networks. Specifically, existing PON systems do not provide an OLT with a single receiver; large capacity, simultaneous wavelength-based transmissions; dynamic and cost-efficient sharing of bandwidth capacity among all ONUs; and scalability and flexibility to incorporate additional ONUs and enable individual upgrades.